Everything you Need to know about Limits

Here’s all you need to know about Limits:

Definition:

In simple terms, the limit of a function at a certain point refers to the value that the function approaches as its independent variable approaches that point. It gives us the ability to describe the behavior of functions at points where they might not be defined, or near points of interest.

Mathematically, for a function \(f(x)\), if for every number \( \epsilon > 0 \) there exists a number \( \delta > 0 \) such that if \( 0 < |x-a| < \delta \) then \( |f(x)-L| < \epsilon \), then the limit of \( f(x) \) as \( x \) approaches \( a \) is \( L \).

This is denoted as:

\[ \lim_{x \to a} f(x) = L \]

Here:

•  \( L \) is the limit value.
•  \( a \) is the point of interest.
•  \( \epsilon \) is any arbitrarily small positive number.
•  \( \delta \) is the distance from \( a \) such that if \( x \) is within this distance, \( f(x) \) will be within \( \epsilon \) of \( L \).

One-Sided Limits:

• Right-hand Limit: The limit as \( x \) approaches \( a \) from the right (values larger than \( a \)).

\(\lim_{x \to a^+} f(x) \)

• Left-hand Limit: The limit as \( x \) approaches \( a \) from the left (values smaller than \( a \)).

\(\lim_{x \to a^-} f(x) \)

For the overall limit \( \lim_{x \to a} f(x) \) to exist, both the left-hand and right-hand limits must exist and be equal.

Common Techniques:

1. Direct Substitution: Plug the value into the function if it’s defined.
2. Factorization: Useful when there’s a common factor in the numerator and denominator causing an indeterminate form.
3. Rationalization: Multiplying the numerator and denominator by a conjugate, typically used for root-based functions.
4. Recognizing Common Limits: Such as \( \lim_{x \to 0} \frac{\sin(x)}{x} = 1 \).
5. Using Squeeze (Sandwich) Theorem: When you can ‘trap’ a function between two others that approach the same limit.

Properties:

1. Linearity: \( \lim_{x \to a} [c \cdot f(x) + g(x)] = c \cdot \lim_{x \to a} f(x) + \lim_{x \to a} g(x) \)
2. Product Rule: \( \lim_{x \to a} [f(x) \cdot g(x)] = \lim_{x \to a} f(x) \cdot \lim_{x \to a} g(x) \)
3. Quotient Rule: If \( \lim_{x \to a} g(x) \neq 0 \), then \( \lim_{x \to a} \frac{f(x)}{g(x)} = \frac{\lim_{x \to a} f(x)}{\lim_{x \to a} g(x)} \)
4. Limits at Infinity: Describes the behavior of a function as \( x \) approaches positive or negative infinity.

Indeterminate Forms:

Sometimes, direct substitution leads to forms that don’t directly give a value but rather a form like \( \frac{0}{0} \) or \( \frac{\infty}{\infty} \). Such forms are called “indeterminate”, and they necessitate further manipulation to determine the actual limit.

Applications:

• Determining the behavior of functions at specific points or as \( x \) goes to infinity.

• Fundamental in defining the derivative (rate of change) and the integral (area under a curve).

To gain a deep understanding of limits, it’s crucial to work through numerous examples, visualize functions graphically, and use the above techniques as needed.